The Historical Record for March 1: Zhang Heng’s Seismometer

Zhang Heng (78-139) was quite the Han dynasty renaissance man. Despite his fame as a poet of considerable talents, a celebrated scholar of the classics, and an official serving at court, Zhang’s greatest and best-known contributions actually came in the field of science and engineering.

He is credited with several accomplishments including a key feature in the clepsydra water clock, improving the Chinese calendar based on his observations and insights into astronomy, and using hydraulic power to rotate an armillary sphere, but Zhang Heng is perhaps most famous for inventing the world’s first seismometer, the 候风地动仪 houfeng didongyi.

According to records from the Han Dynasty, the years 96-125 C.E. were among the most seismically active in China’s history. In that three-decade span, there were 23 major earthquakes in the Han Empire, including several destructive tremblers. Zhang designed his device to help the court detect earthquakes and know (roughly) where in the empire the tremors occurred.

The eminent historian of science in China, Joseph Needham, described Zhang’s seismometer this way:

“It consisted of a vessel of fine cast bronze, resembling a wine-jar, and having a diameter of eight chi (Ed. note: 1 chi/zhi=8 inches). It had a domed cover, and the outer surface was ornamented with antique seal-characters and designs of mountains, tortoises, birds, and animals. Inside there was a central column capable of lateral displacement along tracks in the eight directions, and so arranged (that it would operate) aclosing and opening mechanism.

Outside the vessel there were eight dragon heads, each one holding a bronze ball in its mouth, while round the base there sat eight (corresponding) toads, with their mouths open, ready to receive any ball which the dragons might drop. The toothed machinery and ingenious constructions were all hidden inside the vessel, and the cover fitted down closely all round without any crevice. When an earthquake occurred the dragon mechanism of the vessel was caused to vibrate so that a ball was vomited out of a dragon-mouth and caught by the toad underneath. At the same instant a sharp sound was made which called the attention of the observers.”*

Like a lot of inventers, Zhang first had to prove the skeptics wrong. When he built his device and had it installed at Luoyang, most of the other officals at court scoffed at the notion that Zhang’s contraption would actually work.

But on this date in 138 C.E., an earthquake activated the seismometer which indicated a tremor in the earth to the northwest. A few days later, a runner breathlessly arrived at Luoyang with news of an earthquake and landslides in two prefectures in Longxi (present day Gansu), about 400 miles to the northwest of Luoyang.

Previously, the court could be made aware of a major earthquake only as fast as a messenger could race to the capital from the afflicted area. By indicating the general location of earthquakes, Zhang’s device could shorten the time for aid and assistance to reach disaster zones. After the 138 C.E. Gansu quake demonstrated the value of the seismometer, the task of recording the directions from which tremors originated became part of the regular duties of the Astronomical and Calendar Bureau.

Chinese scientists and historians successfully built a working replica of Zhang’s invention in 1995.

On a related note: Three of the six deadliest earthquakes ever recorded have occurred in China, including the most destructive quake in human history which devastated the province of Shaanxi in 1556 killing an estimated 830,000 people. The other three? Gansu in 1920 and Qinghai in 1927 (both about 200,000) and Tangshan in 1976 (officially 255,000).

*Needham, Joseph, Science and Civilization in China: Volume 3, Mathematics and the Sciences of the Heavens and the Earth cited in Zhang Heng (one of the better Wikipedia articles on Chinese history I’ve come across.)